1. Field of the Invention
The present invention relates to a friction member used in a brake mechanism for a camera shutter such as a focal plane shutter or lens shutter of a camera.
2. Related Background Art
Recently, with an increased demand for an image of higher definition, new video expression, etc., and with the improvement in film sensitivity, it was required to make a shutter speed of a camera higher. In response to this, there has been provided a camera in which a high shutter speed of, for example, 1/8000 sec. and high durability have been attained.
In order to attain such higher shutter speed, it is important not only to make materials for a blade or an arm lightweight, but also to improve the brake performance of a brake mechanism for attenuating an impact when the shutter is halted.
FIGS. 4 and 5 show an example of a driving mechanism of a focal plane shutter in a camera.
In this example, a fore blade interlocking lever 2 and a hind blade interlocking lever 3 disposed on a base plate 1 are driven clockwise around shafts 2a and 3a against springs 4 and 5 with a time lag therebetween. Then, the engagement between a fore and hind blade driving levers 6 and 7 and the blade interlocking levers 2 and 3 are subsequently released so that the fore and hind blade driving levers 6 and 7 are rotated around the shafts 6a and 7a by the force of the springs 8 and 9.
Thus, four shutter blades 12a to 12d (which constitute a first blind or leading shutter curtain 12) connected to the fore blade driving lever 6 and another four shutter blades 13a to 13d (which constitute a second blind or trailing shutter curtain 13) connected to the hind blade driving lever 7 are driven with a predetermined time lag so as to adjust an amount of light passing through an aperture 1a.
As shown in FIG. 6, when the fore blade driving lever is rotated clockwise to reach the end of its moving path, the fore blade driving lever 6 abuts against a brake lever 14 to be subjected to the brake action.
The brake lever 14 eventually abuts against a buffer member 15a so as to stop the fore blade driving lever 6. The hind blade system is operated in the same manner. After the hind blade driving lever 7 abuts against a brake lever 16, the brake lever 16 abuts against a buffer member 15b so as to stop the hind blade driving lever 7.
The torques of the brake levers 14 and 16 are adjusted, for example, by a mechanism which is shown in FIG. 7. In FIG. 7, on a support shaft 17 which supports the brake levers 14 and 16 on the base plate 1, a pair of washers 18 are loaded in such a manner that the brake levers 14 and 16 are sandwiched therebetween. Thus, the washers 18 are urged against the brake levers 14 and 16 by a Belleville spring 19.
When the brake levers 14 and 16 are rotated, a frictional force is generated between the washers 18 and the brake levers 14 and 16 by the urging force of the Belleville spring 19. The rotating energy of the fore and hind blade driving levers 6 and 7 is absorbed by this frictional force.
In the example of FIG. 7, since the brake force varies in accordance with a thickness of the washers 18, one set of washers having proper thickness out of those having several kinds of thickness prepared in advance is selected when the shutter is assembled so as to adjust the brake force to be a predetermined one.
As material for the washers 18, PET (polyethylene terephthalate) is solely used. On friction surfaces between the washers 18 and the brake levers 14 and 16, a small amount of lubricating oil is dropped with the object of improving the abrasion-resistent performance of the friction surfaces and to adjust the frictional force (i.e., brake force).
That is, different from an ordinary brake, the brake levers constitute such mechanism as to mitigate the impact which is generated when the fore or hind blade is stopped, by rotating for a predetermined distance (stroke) while receiving an appropriate brake force.
More specifically, when the brake force is too strong, the brake levers 14 and 16 are not rotated by a predetermined angle so that the impact given to either of the blades becomes large, which leads to a damage or the like of the blade. On the contrary, when the brake force is too week, it loses its function of stopping the brake levers 14 and 16 as a brake. Therefore, it is required to adjust the brake force within a delicate range.
As a result, the lubricant is required in order to adjust the brake force within the delicate range.
Recently, there is increased demand for a super-high speed shutter which has a speed exceeding 1/8000 sec. In order to attain this, it is required to make materials for blades or arms of the shutter to be lightweight and the shutter curtain speed to be more higher.
However, with acceleration of the shutter curtain speed, the impact generated when a shutter blade is stopped becomes larger. Therefore, a brake having a larger absorption energy than a conventional one is required. When the conventional blade material is used as it is, a creak may be generated from the stage on which the shutter has not been used much, or the brake force may be changed or other inconvenient phenomenon may occur. Thus, the initial brake performance may not be maintained, or the lightweight blade or arm may be damaged on an early stage, or the durability may be extremely deteriorated.